Diamond is a preferred material for semiconductor devices because it has semiconductor properties that are superior to conventional semiconductor materials, such as silicon, germanium or gallium arsenide. Diamond provides a higher energy bandgap, a higher breakdown voltage, and a higher saturation velocity than these traditional semiconductor materials.
These properties of diamond yield a substantial increase in projected cutoff frequency and maximum operating voltage compared to devices fabricated using more conventional semiconductor materials. For example, silicon is typically not used at temperatures higher than about 200.degree. C. and gallium arsenide is not typically used above 300.degree. C. These temperature limitations are caused, in part, because of the relatively small energy band gaps for silicon (1.12 eV at ambient temperature) and gallium arsenide (1.42 eV at ambient temperature). Diamond, in contrast, has a large band gap of 5.47 eV at ambient temperature, and is thermally stable up to about 1400.degree. C.
Diamond has the highest thermal conductivity of any solid at room temperature and exhibits good thermal conductivity over a wide temperature range. The high thermal conductivity of diamond may be advantageously used to remove waste heat from an integrated circuit, particularly as integration densities increase. In addition, diamond has a smaller neutron cross-section which reduces its degradation in radioactive environments, that is, diamond is a "radiation-hard" material.
Because of the advantages of diamond as a material for semiconductor devices, there is at present an interest in the growth and use of diamond for high temperature and radiation-hardened electronic devices. In particular, there is a present interest in the growth and use of single crystal diamond as a material for semiconductor devices. This interest is due in part to the increased efficiency of operation of single crystal semiconducting diamond in comparison with polycrystalline semiconducting diamond in which grain boundaries may impede the flow of charge carriers within the device.
Unfortunately, the fabrication of a single crystal diamond film is typically carried out by homoepitaxial deposition of a semiconducting diamond film on a single crystal diamond substrate. Such a single crystal diamond substrate is relatively expensive. Thus, the heteroepitaxial growth of single crystal diamond thin films on nondiamond substrates by chemical vapor deposition (CVD) has long been sought due to its enormous potential impact on the microelectronics industry.
Promising candidate substrate materials for the heteroepitaxial growth of diamond include cBN, .beta.-SiC, BeO, Ni, Cu, Si, and a few refractory metals such as Ta, W and Mo. Nickel is one of the few materials that has a relatively close lattice match with diamond (a=3.52 .ANG. for Ni and a=3.56 .ANG. for diamond). However, apart from reports of heteroepitaxial growth of diamond films on cBN and .beta.-SiC substrates and some limited success of growing some diamond particles oriented on nickel substrates, most experiments have yielded randomly oriented, three-dimensional diamond nuclei which are not suitable for forming a single crystal diamond film as is desirable for semiconductor applications. It is believed that the extremely high surface energy of diamond (in the range of 5.3-9.2 J/m.sup.2 for the principal low index planes) and the existence of extensive interfacial misfit and strain energies between diamond films and nondiamond substrates may be the primary obstacles in forming oriented two-dimensional diamond nuclei.
It has been known for decades that nickel is an effective catalyst metal for diamond crystallization under high pressure and high temperature (HPHT) conditions. See, for example, Preparation of Diamond by Bovenkerk et al., Nature, pp. 1094-1098, Oct. 10, 1959. Although a detailed mechanism of the catalytic effect has not been completely developed, it is believed that its strong reactivity with carbon is essential in the catalytic HPHT diamond growth process. However, nickel's high solubility for carbon and its strong catalytic effect on hydrocarbon decomposition and subsequent graphite formation at low pressures have prevented CVD diamond nucleation on its surface without the deposition of an intermediate graphite layer.
A graphite interlayer generally forms immediately when nickel substrates are placed in a methane-hydrogen CVD environment. This has precluded the possible development of an oriented relationship between the diamond film and the nickel substrate, even though diamond might eventually nucleate and grow on the graphite interlayer. The graphite interlayer also prevents good mechanical adhesion to the underlying substrate.
Sato et al. in an article entitled Epitaxial Growth of Diamond from the Gas Phase, New Diamond Science and Technology, 1987 MRS Int. Conf. Proc., pp. 37-376, discloses some epitaxial diamond growth on a nickel substrate starting with a mechanical surface preparation using a diamond powder to abrade the nickel substrate surface to increase nucleation density. Nucleation density was reported to be highest for a CVD temperature in the range of 800.degree.-900.degree. C. during diamond deposition, although deposition was carried out in the temperature range of 500.degree.-1000.degree. C. Sato et al. disclose that the methane concentration must be kept below a critical value at a given substrate temperature in an attempt to suppress graphite formation.